In Situ Transmission Electron Microscopy Probing of Native Oxide and Artificial Layers on Silicon Nanoparticles for Lithium Ion Batteries

Abstract

Surface modification of silicon nanoparticles via molecular layer deposition (MLD) has been recently proved to be an effective way for dramatically enhancing the cyclic performance in lithium ion batteries. However, the fundamental mechanism of how this thin layer of coating functions is not known, which is complicated by the inevitable presence of native oxide of several nanometers on the silicon nanoparticle. Using in situ TEM, we probed in detail the structural and chemical evolution of both uncoated and coated silicon particles upon cyclic lithiation/delithation. We discovered that upon initial lithiation, the native oxide layer converts to crystalline Li2O islands, which essentially increases the impedance on the particle, resulting in ineffective lithiation/delithiation and therefore low Coulombic efficiency. In contrast, the alucone MLD-coated particles show extremely fast, thorough, and highly reversible lithiation behaviors, which are clarified to be associated with the mechanical flexibility and fast Li(+)/e(-) conductivity of the alucone coating. Surprisingly, the alucone MLD coating process chemically changes the silicon surface, essentially removing the native oxide layer, and therefore mitigates side reactions and detrimental effects of the native oxide. This study provides a vivid picture of how the MLD coating works to enhance the Coulombic efficiency, preserves capacity, and clarifies the role of the native oxide on silicon nanoparticles during cyclic lithiation and delithiation. More broadly, this work also demonstrates that the effect of the subtle chemical modification of the surface during the coating process may be of equal importance to the coating layer itself.

In situ methods for Li-ion battery research: A review of recent developments

“…It conducts Li-ions and mechanically confines pulverized particles, thereby preventing loss of active material. This finding clarifies some of the underlying principles of coating active battery materials with metal oxides such as Al 2 O 3 , which is a well-known strategy to improve the overall battery performance [52,53].…”

Chemomechanical modeling of lithiation-induced failure in high-volume-change electrode materials for lithium ion batteries

“…Some coating materials, such as Al 2 O 3 , 43,108,109 function as a passivation layer, which suppresses unwanted chemical reactions between electrodes and electrolytes, and subsequently prevents wasting Li sources in the electrolyte. Conductive coatings such as carbon, [110][111][112][113][114][115][116][117][118][119][120] metals, [121][122][123] and conductive polymers 43,[124][125][126][127] can enhance the redox reaction kinetics and improve the current collection efficiency. Surface coatings can also prevent electrochemical welding between particles of active materials.…”

Investigation of the Electron Transfer at Si Electrodes: Impact and Removal of the Native SiO2Layer

“…Although only Li + electrolyte is used in this study, the lithiation is not investigated, since the potential of all Si electrodes was kept at open circuit (E S = 3.2 V) and the first lithiation requires E S ≤ 0.2 V. 26 In contrast to pristine graphite electrodes, pristine Si electrodes are covered by a native SiO 2 surface layer, which is at least 1-3 nm in thickness and possess passivating properties for many charge transfer reactions. [27][28][29] Undoped Si/SiO 2 has been used as insulating substrate for the characterization of single-walled carbon nanotubes 30,31 and gapped Au nanobands 32 by SECM. For these examples the insulating property of undoped Si/SiO 2 substrates is beneficial.…”